I use Trillian, and it has been good to me. Only once have I had issues with it ... and that was when, for some reason, this one contact of mine would message me from his phone. If he was on AIM from his PC, it was fine, but as soon as he IMed me from his phone, boom. Crash. Never did figure out what it was.

I use Trillian - it takes a while to get used to, but once I did, I've never looked back. It allows for multiple usernames at once, multiple IM programs at once, and it keeps every nicely color-coded. It's got its flaws, but for its purpose, it does a spectacular job.